Firing sol-gel alumina particles

ABSTRACT

Sol-gel alumina that is dried but unfired can be explosively comminuted by feeding the dried gel into a furnace held at temperatures above those at which vaporizable materials are eliminated from the particles of gel. At suitably elevated temperatures the firing is sufficient to form fully densified alpha alumina particles of a size suitable for direct use as abrasive grits.

BACKGROUND OF THE INVENTION

This invention relates to aluminous abrasive grits and particularly tosol-gel alumina abrasive materials with improved grinding performance.

Sol-gel alumina abrasives are conventionally produced by drying a sol orgel of an alpha alumina precursor, (which is usually but notessentially, boehmite), at about 125° to 200° C. to remove the watercomponent of the gel; breaking up the dried gel into particles of thedesired size for abrasive grits; perhaps calcining the particles,(generally at a temperature of from about 400°-800° C.), to form anintermediate form of alumina; and then finally firing the calcinedpieces at a temperature sufficiently high to convert them from anintermediate form such as gamma alumina to the alpha alumina form.Simple sol-gel processes are described for example in U.S. Pat. Nos.4,314,827; 4,518,397; 4,881,951 and British Patent Application2,099,012.

In a particularly desirable form of sol-gel process, the alpha aluminaprecursor is "seeded" with a material having the same crystal structureas, and lattice parameters as close as possible to, those of alphaalumina itself. The "seed" is added in as finely divided form aspossible and is dispersed uniformly throughout the sol or gel. It can beadded ab initio or it can be formed in situ. The function of the seed isto cause the transformation to the alpha form to occur uniformlythroughout the precursor at a much lower temperature than is needed inthe absence of the seed. This process produces a crystalline structurein which the individual crystals of alpha alumina, (that is those areasof substantially the same crystallographic orientation separated fromadjacent crystals by high angle grain boundaries), are very uniform insize and are essentially all sub-micron in diameter. Suitable seedsinclude alpha alumina itself but also other compounds such as alphaferric oxide, chromium suboxide, nickel titanate and a plurality ofother compounds that have lattice parameters sufficiently similar tothose of alpha alumina to be effective to cause the generation of alphaalumina from a precursor at a temperature below that at which theconversion normally occurs in the absence of such seed. Examples of suchseeded sol-gel processes are described in U.S. Pat. Nos. 4,623,364;4,744,802; 4,954,462; 4,964,883; 5,192,339; 5,215,551; 5,219,806 andmany others.

The optional calcining of the dried sol-gel is often preferred so as tominimize the time needed at the elevated firing temperatures. This isbecause the firing operation performs the tasks of converting thetransitional alumina forms to the alpha form and the sintering of thealpha alumina to close up residual porosity and ensure that theparticles have adequate density and hardness to function well asabrasive grits. It is known that excessive time at sinteringtemperatures, which are generally between 1300° and 1400° C. for seededsol-gel materials and about 100° C. higher than that for unseededsol-gel aluminas, can lead to crystal growth. Since crystal growth isgenerally regarded as undesirable, it is considered appropriate to carryout the calcining separately and so minimize the time at such elevatedtemperatures. This procedure is followed in spite of the extra cost ofmaintaining two high temperature operations.

Since the drying operation is followed by a crushing and screeningoperation the grain is reduced to room temperature and the heat used todry the grain is given up to the surroundings. This is of course veryinefficient.

The crushing operation is performed after the drying because, at thispoint the material is relatively easily broken up. If it were left tillafter the firing operation, the material would be so hard that excessiveamounts of energy would be required. It is therefore common-sense tocrush at the pre-fired stage. In addition it is considered that firingwill be more efficient since the particles will more rapidly reach thefiring temperature in the furnace if they are small.

It has now been found possible to significantly reduce the energyconsumption involved in the production of alumina by a sol-gel process.This is achieved by a manipulation of the process in a manner that iscompletely contrary to the intuitive reasoning used in designingconventional processes. The novel process produces alpha aluminaparticles in a very desirable form that is fully densified and welladapted to use in abrasive applications. Moreover the system is flexibleenough to permit design of the abrasive grits obtained.

GENERAL DESCRIPTION OF THE INVENTION

The process of the invention comprises feeding a dried but not firedsol-gel alumina having a volatilizable content of at least 5% by weight,directly into a furnace held at a temperature above 400° C. andcontrolling the temperature and residence time to produce an explosivelycomminuted alumina. Under certain conditions when the temperature in thefurnace is high enough and the time therein is long enough, the sol-gelalumina can be converted directly to the alpha alumina form and sinteredto a density that is at least 95% of the theoretical density.

Sol-gel alumina generally dries to form lumps a few millimeters in sizeand this is basically dried boehmite, each molecule of which has anassociated molecule of water, with perhaps some residual water notcompletely removed in the drying. In addition advantageous modifierssuch as magnesia, yttria, rubidia, caesia, or rare earth or transitionmetal oxides are often added the sol-gel in the form of their solublenitrates and these too will contribute volatilizable components, (suchas nitrogen oxides), to the dried gel. If an acid such as nitric oracetic acid has been used to peptize the sol-gel there may also beresidues of this acid in the dried sol-gel. Generally the dried gel hasan overall vaporizable content of from about 5 to about 50%, preferablyfrom about 10 to about 45%, and more preferably from about 20 to about40% by weight. Drying is usually conducted at a temperature below about200° C. and more usually at temperatures far lower. For this reason thedried gel contains substantial amounts of vaporizable material when itis charged into the furnace.

While the invention is primarily directed towards the explosivecomminution of dried sol-gel materials, these materials can also includeother components that do not themselves comprise any volatilizablematerial. Thus it is possible to include in the sol-gel materialcomponents such as alpha or gamma alumina powder, silicon carbide (bothparticulate and whisker forms), zirconia, cubic boron nitride, and otherabrasive materials, providing the overall content of vaporizablematerial in the dried mixture remains above the 5% by weight material.

When the lumps of dried gel are placed in the furnace the vaporizablematerial in the lumps expands explosively causing them to fly apartleaving smaller particles which are highly suitable for use in grindingapplications. If the residence time in the furnace is sufficiently long,the smaller particles that remain are rapidly converted to the alphaphase and sinter very quickly to the essentially fully densified form.The violent nature of this process has led to it being describedfamiliarly as "explosive comminution" though in a preferred embodimentthe process goes beyond comminution and includes firing to the alphaphase and, in some cases sintering to essentially theoretical density.Where the temperature is somewhat lower, the amount of comminution mayreduced somewhat and may chiefly result in the break-up of the largerpieces and the creation of weakness lines in the remaining pieces thatrender them easily broken up in a subsequent comminution operation. Thishowever is also regarded as explosive comminution.

Therefore "explosively comminuted" material is understood to be formedwhen dried sol-gel alumina particles are fed into the furnace and are atleast partially broken into smaller particles without the use of anyexternally imposed force.

When the furnace residence time is relatively short or the furnacetemperature is relatively low, the sintering process and even theconversion to the alpha phase may not be completed when the materialexits the furnace. In such event some or all of the particles may beporous to some degree and these relatively loosely consolidated largerparticles may be broken up by a light milling operation before beingsintered to a density in excess of 95% of theoretical in a separatefurnace or by a second passage through the rotary furnace. This issometimes preferred since a very intense explosive comminution can leadto the production of significant amounts of very fine particles that maybe less useful in some abrasive applications. The less severe explosivecomminution has the effect of making even apparently unbroken particlesvery easy to comminute in a subsequent operation. Alternatively andsometimes preferably, the fired material that has not been completelyexplosively comminuted, which often has a degree of porosity, may be atleast partially impregnated with a volatilizable liquid such as waterand passed once more through the rotary furnace to complete thecomminution process.

Adjustment of the firing conditions to produce a product that is porousas described above also affords the opportunity of impregnating theporous material with solutions of modifying agents such as for examplean aqueous solution of a soluble salt of magnesium, yttriium, atransition element, rubidium, caesium or a rare earth metal. Onsintering these materials will usually generate the modifying oxide in avery effective form and simultaneously generate further amounts ofvolatilizable material that can be used to bring about explosivecomminution.

Abrasive grain prepared in the above way often has an unexpectedlybetter grinding performance than grain obtained by more conventionalprocesses. It is theorized that this may be because the comminutiontechnique does not impose physical strains on the material of the typethat could give rise to defects in the abrasive grit structure.Regardless of theory this performance improvement is surprising andsignificant.

DRAWINGS

FIG. 1 shows a DTA trace of a seeded sol-gel alumina.

FIG. 2 shows a simplified elevation of an apparatus adapted for theimplementation of one embodiment of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

From the drawing presented as FIG. 1, which is a Differential ThermalAnalysis trace following a seeded sol-gel as its temperature is raised,it will be seen that there is an endotherm at about 400° C. Thisindicates the loss of volatiles including water and acid and saltdecomposition products. It is this loss of volatiles that causes theexplosive comminution. Clearly the faster this loss occurs, the moreexplosive the decomposition will be. By about 600° C. the amount ofvolatiles to be removed has significantly diminished and conversion tothe anhydrous phases of alumina such as gamma alumina is complete. Athigher temperatures still, the conversion to the alpha phase begins.With seeded sol-gel materials, this occurs at about 1150° C. or evenlower. This is indicated by the peak in FIG. 1. In an unseeded sol-gel,the trace will be very similar except that the alpha conversion peakwill occur at a rather higher temperature, perhaps 1250° C. or so.

To practice the invention it is only necessary to heat at a temperatureat which the volatiles begin to be driven off. Clearly highertemperatures than the minimum favor a very rapid decomposition that hasthe maximum explosive effect. However if the heating is sufficientlyrapid even modest temperatures at the lower end of the above ranges canbe used effectively.

If temperatures at the lower end of the above ranges, (that is wherealpha alumina has still not been formed), are used, the explosivelycomminuted material must be subjected to a further firing operation tocomplete the conversion to the alpha phase and (if desired) sinter thematerial to essentially theoretical density, (generally taken to be inexcess of 95%). While this involves further expense, it does allow theuse of rotary furnace materials that are much more sturdy and far lessexpensive that the silicon carbide tubes that are standard for furnacesin which all operations are performed at the same time.

The sol-gel alumina is typically dried at temperatures below about 200°C. and more preferably at much lower temperatures such as from about 75to about 175° C.

As has been indicated above, it is highly desirable to provide that thelarge particles of dried sol-gel material are heated as rapidly aspossible to achieve the maximum expansion and explosive comminution. Theapparatus illustrated in simplified elevation and partial cross-sectionin FIG. 2 is well suited to meet these requirements. Uncrushed driedparticles of a sol-gel alumina about 0.5 to about 1 cm in diameter arefed into the hopper, 1, from which they are fed through a vibratoryfeeder, 2, to a secondary feeder, 3. This secondary feeder dischargesparticles into an air eductor, 4, which in turn accelerates theparticles using a stream of compressed air entering through port, 5,which carries the particles through a conduit, 6, and into a rotaryfurnace, 7, having upper and lower ends, at a point, 8, adjacent the hotzone within the furnace. In use the particles explode when they enterthe hot zone and comminuted particles exit the lower end, 9, of thefurnace.

In an explosive comminution process the heating of the lumps of driedgel is preferably done rapidly to achieve the maximum explosive effect.While several furnace designs other than that illustrated in FIG. 2could be adapted to meet this requirement, a highly suitable furnace forcarrying out the process is a rotary furnace comprising a tube inclinedat an angle to the horizontal and rotating about its axis, said tubebeing heated by externally applied heat. The rotation of the tubeensures that the lumps or particles inside the tube are in constantmovement such that no one part of a lump or particle is heated bycontact with the tube to the exclusion of another part. The speed ofrotation and the angle of incline of the tube determine the residencetime inside the furnace. These parameters are preferably adjusted toensure that the evaporation of the vaporizable materials from inside thelumps happens rapidly rather than gradually. This is to enable theparticles formed after the explosive breakup of the lumps to spend themaximum time firing and densifying.

Other furnace designs can be used as desired including batch furnacesoptionally with fluidized beds and furnaces with microwave or inductionheating.

A rotary furnace for use with firing temperatures of the order of thoseneeded to sinter alumina conveniently has a silicon carbide tube. Thisis because of its ability to stand up to the. physical demands of theprocess including the temperature variations along the length and thedifferent loads at different point along the tube length. Siliconcarbide is also able to withstand any acidic gases that might begenerated, for example as nitrate residues are eliminated. If however itis intended to carry out the explosive comminution and conversion to thealpha form at temperatures below those at which full sintering occurs,it is possible to use metal alloys capable of withstanding temperaturesof up to about 1200° C. such as "Inconel".

Using a rotary furnace, the process of the invention requires aresidence time in the hot zone of from about 1 second to about 30minutes and preferably from about 2 seconds to about 20 minutes. Toachieve such residence times the angle of elevation of the tube ispreferably from about 1° to about 60° and more preferably from about 3°to about 20° and the speed of rotation is preferably about 0.5 to about20 rpm and more preferably from about 1 to about 15 rpm.

When firing a seeded sol-gel alumina the firing temperature in the hotzone of a rotary furnace is usually from about 400° C. to about 1500° C.and more preferably from about 600° C. to about 1400° C. For an unseededsol-gel alumina the hot zone is preferably maintained at a temperatureof from about 400° C. to about 1650° C. and more preferably from about600° C. to about 1550° C.

The particles obtained by the explosive comminution process of theinvention tend to have pronounced aspect ratios, that is, they have onedimension that is substantially longer than any other. Such particlesare particularly useful in coated abrasive applications.

The process of the invention is applicable to all types of sol-gelparticle production particularly where these are intended for abrasiveapplications. The sol-gel can be seeded or unseeded, the only differencein the conditions used is that a higher sintering temperature isgenerally required when the sol-gel is unseeded.

Because the process of the invention permits the elimination of thephysical comminution stage typical of the prior art, the dried gel canbe fed directly into the furnace from the drier. This saves considerabletime and energy costs.

DESCRIPTION OF PREFERRED EMBODIMENTS

The process of the present invention is now described with particularreference to the firing of a seeded sol-gel alumina in a rotary furnace.These examples are for the sake of illustration only and are intended toimply no essential limitations on the essential scope of the invention.

Example 1

A Ross mixer was charged with 74,657 gm of deionized water and a slurryof alpha alumina seeds having a BET surface area of about 120m² /gm madeby adding 6,000 gm of a 6% slurry of the seeds in deionized water to10,000 gm of deionized water. Boehmite, ("Disperal" sold under thattrademark by Condea GmbH), in an amount of 36.00 kg was also added andthe mix was evacuated and agitated for 5 minutes. A solution of 1,671 gmof 70% nitric acid in 5,014 gm of deionized water was then added whilethe stirred mixture was maintained under vacuum and stirred for afurther 5 to 10 minutes. The vacuum was then released and the mixturewas gelled by passing the mixture through an in-line mixer-homogenizerwhile injecting into the mixture a solution of 1,671 gm of 70% nitricacid in 5,014 gm of deionized water.

The gel was dried and broken up into lumps of from about. 0.25cm to 1 cmin size and these lumps were fed into a furnace. The dried sol-gel lumpswere fed directly into a rotary furnace comprising a silicon carbidetube 213 cm in length and 15 cm in diameter, with a 50 cm hot zonemaintained at 1405° C. The tube was inclined at 6° to the horizontal androtated at about 18 rpm.

The lumps were explosively comminuted to a range of particle 20 sizesfrom which 50 T sized grits were separated for physical testing. Thetime for the material fired to transit the rotary furnace was about 1 to2 minutes. The fired grits had a density in excess of 3.8 gm/cc andcomprised microcrystallites of alumina about 0.2 micron in diameter.

For the sake of comparison the same sol-gel formulation was dried in thesame way, roll-crushed to produce -24 mesh particles which were thencalcined at about 800° C. before being fired in a conventional manner ina conventional rotary furnace.

The two samples were then made up into abrasive belts using exactly thesame amounts of grit, backing, maker and size coats. Each belt carried590 gm of grit per square meter of surface area and was 6.4 cm wide and152.4 cm long. The belts were run at 9,000 surface meters per minute andwere used to cut a 304 stainless steel bar for 4 minutes under a watercoolant at an applied force of 6.8 kg.

The belt made using the conventional grits cut 74 g during this periodwhile the belt made with the explosively crushed grits cut 94 g, or a27% improvement over the conventional belt.

Example 2

Dried lumps of seeded sol-gel alumina at room temperature with a size ofabout +24T were fed directly at a rate of about 2.25 to about 4.5kg/hour into the hot zone of a rotary furnace maintained at 1000° C.using an apparatus substantially as described in FIG. 2. The furnace wasthe same as was used in Example 1 except that the tube was rotated atabout 10 rpm and was inclined at about 7° to the horizontal. The gelparticles were explosively comminuted in the furnace and the particlesize distribution was as described in Table 5 below.

                  TABLE 1                                                         ______________________________________                                        SIZE RANGE   AMOUNT IN RANGE                                                  ______________________________________                                        +30          41%                                                              -30 + 40     31%                                                              -40 + 50     11%                                                              -50 + 60      3%                                                              -60           4%                                                              ______________________________________                                    

In a separate operation the above explosively comminuted material wasfurther sintered to a density greater than 3.8 gm/cc and the size rangeof the sintered material was as shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                        SIZE RANGE   AMOUNT IN RANGE                                                  ______________________________________                                        + 30         22%                                                              -30 + 40     38%                                                              -40 + 50     23%                                                              -50 + 60      9%                                                              -60           8%                                                              ______________________________________                                    

What is claimed is:
 1. A process for explosively comminuting acomposition comprising a dried but unfired sol-gel alumina saidcomposition having a volatilizable content of at least 5% by weight,which comprises feeding particles of the composition directly into afurnace held at a temperature from about 400° C. to 1600° C. andcontrolling the residence time in the furnace to produce explosivelycomminuted alumina particles.
 2. A process according to claim 1 in whichthe furnace is a tubular rotary furnace.
 3. A process according to claim2 in which the tube is inclined at an angle of from 1 to about 60° tothe horizontal.
 4. A process according to claim 3 in which the tube isrotated at from about 0.5 to about 40 rpm.
 5. A process according toclaim 1 in which the residence time in the zone of maximum temperaturein the furnace, (the hot zone), is from about 1 second to about 30minutes.
 6. A process according to claim 1 in which the dried sol-gelalumina containing composition fed into the furnace comprises from about5 to about 60% by weight of volatilizable material.
 7. A processaccording to claim 1 in which the dried sol-gel alumina containingcomposition is fed directly into the vicinity of the hot zone of thefurnace.
 8. A process according to claim 1 in which the temperature andresidence time in the furnace are sufficient to form the alpha phase andsinter to a density of at least 95% of theoretical.
 9. A processaccording to claim 1 in which the furnace is maintained below thetemperature required to sinter the alumina, and the explosivelycomminuted material is subsequently sintered to essentially theoreticaldensity.
 10. A process according to claim 9 in which the explosivelycomminuted material is subjected to a crushing process to adjust theparticle size further before it is sintered.
 11. A process according toclaim 9 in which the explosively comminuted material is treated with asolution of a soluble salt of a metal selected from the group consistingof rare earth metals, transition metals, rubidium, caesium and yttriumbefore being subjected to the sintering operation.
 12. A process forexplosively comminuting a composition comprising a dried but unfiredseeded sol-gel alumina, said composition comprising from about 20 toabout 40% by weight of volatilizable material which comprises feedingthe composition into a tubular rotary furnace having a hot zonemaintained at a temperature of from about 600° C. to about 1500° C. inwhich the tube is inclined at an angle of from about 2° to about 20° tothe horizontal and rotated at from about 2 to about 20 rpm.
 13. Aprocess according to claim 12 in which the temperature and residencetime in the furnace are sufficient to form the alpha phase and sinter toa density of at least 95% of theoretical.
 14. A process according toclaim 12 in which the furnace is maintained below the temperaturerequired to sinter the alumina, and the explosively comminuted materialis subsequently sintered to essentially theoretical density.
 15. Aprocess according to claim 14 in which the explosively comminutedmaterial is subjected to a milling process to adjust the particle sizefurther before it is sintered.
 16. A process according to claim 14 inwhich the explosively comminuted material is treated with a solution ofa soluble salt of a metal selected from the group consisting of rareearth metals, transition metals, rubidium, caesium and yttrium beforebeing subjected to the sintering operation.
 17. A process forexplosively comminuting a dried but unfired unseeded sol-gel aluminacomprising from about 20 to about 40% of volatilizable material whichcomprises feeding the dried. sol-gel into a tubular rotary furnacehaving a hot zone maintained at a temperature of from about 600° C. toabout 1650° C. in which the tube is inclined at an angle of from about3° to about 20° to the horizontal and rotated at from about 1 to about20 rpm.
 18. A process according to claim 17 in which the temperature andresidence time in the furnace are sufficient to form the alpha phase andsinter to a density of at least 95% of theoretical.
 19. A processaccording to claim 17 in which the furnace is maintained below thetemperature required to sinter the alumina, and the explosivelycomminuted is subsequently sintered to essentially theoretical density.20. A process according to claim 19 in which the explosively comminutedmaterial is subjected to a milling process to adjust the particle sizefurther before it is sintered.
 21. A process according to claim 19 inwhich the explosively comminuted material is treated with a solution ofa soluble salt of a metal selected from the group consisting of rareearth metals, transition metals, rubidium, caesium and yttrium beforebeing subjected to the sintering operation.